1. Field of the Invention
This invention is in the field of computer generated raster graphics and, more particularly, relates to methods and apparatus for determining between two types of displays, alphanumeric or graphic, which shall control the intensity and color of each picture element of the reaster of a cathode ray tube.
2. Description of the Prior Art
Alphanumeric raster scan CRT displays form a principal communication link between computer users and their hardware/software systems. The basic display device for computer generated raster graphics is the CRT monitor, which is closely related to the standard television receiver. In order for the full potential of raster graphics to be achieved, such displays require support systems which include large-scale random-access memories and digital computational capabilities. As the result of recent developments of large-scale integrated circuits, the price of digital memories has been reduced significantly and computers in the form of microcomputers are available which have the capability of controlling the displays at affordable prices. As a consequence, there has been a surge of development in raster graphics. Typically, each pixel in a rectangular array of the picture elements of a CRT is assigned a unique address, comprising the x and y coordinates of each pixel in the array. Information to control the display is stored in a random-access frame memory (RAM) at locations having addresses corresponding to those assigned to the pixels. The source of pixel control data stored in the RAM is typically a microcomputer located in a graphic controller which will write into the addressable memory locations the necessary information to determine the type of display. This frequently is an address in a color look-up memory, at which location there is stored the necessary binary color control signals to control the intensity of the color of each pixel of an array. The horizontal and vertical sweep of the raster scan is digitized to produce the addresses of the pixels, which addresses are applied to the frame memory in which the controller has previously written the information determinative of the display. This information can be an address in a color look-up memory. The data is read out of the addressed location in the color look-up memory and the necessary color control signals are obtained. The color signals are converted to analog signals and applied to the three color guns of the typical CRT to control the intensity and color of each pixel as it is scanned.
There are basically two kinds of displays that can be produced by a raster graphics system, one being an alphanumeric type display in which alphanumeric symbols are displayed in cells of uniform size, and the other being a graphic type display in which the color and intensity of each pixel is uniquely determined, and which is used for drawing lines and geometric figures, for example.
It is frequently the case that for each pixel of the array there is stored in the RAM of the raster graphic system, sometimes called the frame memory, information for both an alphanumeric type or mode of display and a graphic type or mode of display. Prior to this invention, there has been no way in which the priority of type, or mode, of display, the color and intensity of each pixel of the raster, is controlled on a pixel by pixel basis where two or more types of display information have been written into the frame memory for one or more pixels.